This invention relates to optical recording media, and more particularly, to optical recording media having a liquid crystal layer.
In these years, great activities are concentrated on the development of erasable, rewritable optical recording media. One known example of such optical recording media utilizes the phase transition of liquid crystal [see SPIE, 420, 194-199 (1983)]. In this typical example, liquid crystal, typically smectic liquid crystal is sandwiched between a light absorbing electrode layer on a substrate and a transparent electrode layer on a transparent substrate. A laser beam is directed to the medium from above the transparent substrate under a certain electric field across the electrodes and the light absorbing electrode layer then absorbs radiation to produce heat. Phase transition, for example, for smectic to nematic takes place in the exposed spot at an elevated temperature. Upon removal of the beam, the heated spot is quenched so that the phase transition is kept unchanged. A recorded spot is formed in this way to complete writing. Read-out may be carried out by directing a reading laser beam to the recorded medium where light scattering occurs at recorded spots.
Erasing may be carried out by applying an electric field across the electorodes of the recorded medium or heating and gradual cooling, thereby inducing phase transition back to the original phase, for example, smectic phase. Alternatively, erasing operation may be carried out by applying a laser beam under a certain voltage applied across the electrodes or by heating without any voltage. The medium is ready for writing, that is, rewritalble.
Preferably, the liquid crystal layer contains a dichroic dye. A layer of aligner may be provided between the electrode layer and the liquid crystal layer to assist in the alignment of liquid crystal molecules.
These media generally use a light-absorbing electrode layer formed of metal, for example, Cr, Cr alloy, Au, Al, etc. which is rather less sensitive to light, resulting in some disadvantages including low recording sensitivity and low S/N ratio upon reading out.
The inventors have found that nematic and smectic liquid crystals are not fully satisfactory in directional orientation or sensitivity and S/N ratio. There is a need for further improving the sensitivity, S/N ratio, storage life, and other properties of the liquid crystal layer of optical recording media.